Corrosion resistance before and after coating of the metal material has generally been improved by using a surface-treated metal material having a zinc phosphate or a chromate coating. A zinc phosphate coating is capable of improving the corrosion resistance of steel materials such as a hot-rolled steel strip and a cold-rolled steel strip, galvanized steel strip, and some aluminum alloys.
However, the surface treatment used in forming such a zinc phosphate coating is associated with the inevitable generation of sludge which is the by-product of the reaction, and the corrosion resistance after coating had been insufficient in some steel materials such as a high tensile strength steel strip and some aluminum alloys.
Galvanized steel strips and aluminum alloys can also be provided with sufficient performance after coating by forming a chromate coating on such a material.
However, in consideration of current environmental regulation, use of the chromate treatment which inevitably includes toxic hexavalent chromium in both the treatment solution and the coating layer formed by such treatment is gradually avoided. Because of such a situation, various methods including the methods as described below have been proposed as a method capable of providing a coating layer free from toxic components by a surface treatment.
For example, Patent Document 1 discloses a compound containing a nitrogen atom having a lone pair, and a chromium-free coating composition for a metal surface containing this compound and a zirconium compound. This method discloses the application of the compound to provide a coating containing no harmful hexavalent chromium and having an improved corrosion resistance after coating as well as a satisfactory adhesion.
Similarly, many chemical conversion methods such as those disclosed in Patent Documents 2 to 5 were proposed as surface-treatment methods for depositing a coating exhibiting an excellent adhesion after the coating as well as an excellent corrosion resistance.
Patent Document 6 discloses a composition for surface treatment of a metal containing a metal acetylacetonate and at least one compound selected from a water-soluble inorganic titanium compound and a water-soluble zirconium compound at a weight ratio of 1:5000 to 5000:1.
Patent Document 7 discloses a surface-coated metal material having an excellent corrosion resistance produced by forming on the surface of a metal material a corrosion-resistant coating containing an oxide of at least one element selected from the group consisting of Ti, Cr, Nb, Ta, Al, Si, and Zr and a carbide of at least one element selected from the group consisting of Ti, V, Al, Cr, Si, W, Ta, Fe, and Zr in a total content of at least 10% by weight. Patent Document 7 also teaches that a metal material having an excellent corrosion resistance can be provided by this coating.
Patent Document 1 JP 2000-204485 A
Patent Document 2 JP 56-136978 A
Patent Document 3 JP 8-176841 A
Patent Document 4 JP 9-25436 A
Patent Document 5 JP 9-31404 A
Patent Document 6 JP 2000-199077 A
Patent Document 7 JP 7-228961 A